Inorganic nanoparticles, nanoclusters, and colloids have become a subject of intensive research and offer a great many potential uses if their size, ligand sphere, and positioning can be reliably controlled (Shipway A. N. et al., ChemPhysChem. 1: 18-52 (2000)). A variety of devices can be envisioned, ranging from specialized nanosensors to molecular electronics and nanoscale optical devices. Many such applications are not presently practical due to the lack of appropriate methods for synthesis of nanoparticle chains and for fabrication of nanoparticle chains into a circuit.
Numerous approaches to synthesis of nanoparticles exist, including pyrolysis of organometallic precursors, arrested precipitation, precipitation in reverse micelles, and exchange (metathesis) reactions. Because nanoparticle properties depend strongly on size, shape, crystallinity, and surface derivatization, the particle synthesis is normally tailored to control these parameters for a particular application. In general, if the nanoparticles are intended to be utilized in their native particulate state without any fusion into bulk material (agglomeration), then any synthetic method yielding appropriate size control and crystallinity may be utilized (Jacobson et al, U.S. Pat. No. 6,294,401 (2001)).
Because of this, current synthetic methods for nanoparticles are largely concerned with obtaining size control and a viable synthesis of the desired compound (Schmid et al., Adv. Mater. 10: 515-526 (1998)). Syntheses have been designed to incorporate ligands with supramolecular functionality, with the aim of connecting one nanoparticle to another molecular entity or nanoparticle (Loweth et al., Angew. Chem. Int. Ed. 38, 1808-1812 (1999); Boal et al., J. Amer. Chem. Soc. 122: 734 (2000); Liu et al., Adv. Mater. 12: 1381-1383 (2000); Mann et al., Adv. Mater. 12: 147 (2000); Novak et al., J. Amer. Chem. Soc. 122: 3979-3980 (2000)). For example, monofunctional gold nanoparticles have been produced by statistical ligand exchange reactions, which is a very difficult task requiring subsequent extensive purification and separation steps, such as high-performance liquid chromatography (U.S. Pat. No. 5,360,895, Hainfeld et al. (1994), U.S. Pat. No. 5,521,289, Hainfield et al. (1996), U.S. Pat. No. 6,121,425, Hainfield et al. (2000)).
Biological techniques have been found to be useful in directing synthesis of inorganic materials (Storhoff et al., Chem. Rev. 99: 1849-1862 (1999); Lee et al., Science, 296, 892-895 (2002)). The realm of biology offers examples of both controlled nanoparticle synthesis and the building of elaborate functional structures by the use of polymers. For example, ferritin is a cage-like nanoparticle of a specific size that can be synthesized in a controlled fashion. Ferritin and similar structures have been used in the synthesis of nanoparticles of well-controlled size (Mukherjee et al., Angew. Chem. Int. Ed. 40: 3585 (2001); Shenton et al., Angew. Chem. Int. Ed. 40: 442-445 (2001)). Biology also offers a number of diverse processes that can be carried out by polymeric chains such as, for example, peptide and nucleotide chains. Attempts have also been made to utilize biological motifs to control the relative positioning of nanoparticles (Lee et al., Science, 296, 892-895 (2002)).
Nanoparticles fall into two general categories: charge-stabilized colloids and ‘molecularly’ soluble colloids/chemical entities. Charge-stabilized colloids are typically synthesized in polar media. Although charge-stabilized colloids are thermodynamically unstable due to high surface energy, they maintain their small size by electrostatic repulsion. Kinetically, charge-stabilized colloids are very unlikely to agglomerate.
Agglomeration of molecularly soluble nanoparticles can typically be avoided by modifying the entropy, solvation energy, and/or steric shielding of the nanoparticles. These modifications are generally accomplished by the use of organic ligands, which allows fine-tuning of solubility for various solvents. The bond strength of the ligands to a nanoparticle typically varies from low strength Lewis acid-Lewis base interactions to higher-strength covalent bonds.
Most ligands are quite mobile within the ligand sphere of a nanoparticle and can migrate from one side of a nanoparticle to another. Therefore, while two ligands may be on opposite sides of a nanoparticle initially, they can migrate to the same side of the particle over time, especially when there is an attractive interaction (Boal et al., J. Amer. Chem. Soc. 122: 734 (2000). Migration of ligands can interfere with building complex supramolecular structures out of nanoparticles. Further, because the ligand sphere is not rigid, the ligand-particle-ligand ‘bond angle’ is not fixed for any two ligands on the particle. As a result, the ligands are free to move around, which can destroy the desired supramolecular effect.
What has been needed, therefore, are generalized coupling chemistries that allow buildup of arbitrary chains of nanoparticles in a polymeric fashion. Methods of nanoparticle synthesis are therefore needed that allow for the reliable incorporation into the nanoparticle ligand sphere of functionality through specifically designed chemically reactive sites.